Laser cutting method and apparatus for optical fibres or waveguides

ABSTRACT

A cutting method and apparatus are provided to cut a portion of an optical fibre or waveguide with a laser beam. The proposed cutting action takes advantage of the sharp cutting edge of a laser beam generated so as to have a predetermined asymmetric intensity distribution. In operation, a prescribed amount of the beam distribution is impinged on the fibre or waveguide portion and causes the portion to ablate or vaporise so as to effect a cut at the portion in dependence upon the impingement. No translation of the laser beam across the fibre or waveguide is effected during cutting. The proposed cutting action bears definite advantages over conventional cutting techniques and finds utility for many optical fibre or waveguide applications. For example, the proposed cutting action can produce a substantially flat optical fibre or waveguide surface or a lens of enhanced quality at the end of the fibre or waveguide portion.

FIELD OF THE INVENTION

This invention relates to a laser cutting method and apparatus and moreparticularly, but not exclusively, concerns a cutting method andapparatus utilising a laser in optical fibre and optical waveguideapplications.

BACKGROUND OF THE INVENTION

The state of the art to which the present invention relates is presentedhereinafter in three parts, namely, in relation to: (1) the knowntechnique of mechanical cleaving of optical fibres and opticalwaveguides, (2) the known techniques for producing lens-shapes onoptical fibres, and (3) the known techniques for cutting optical fibreswith a laser.

1. Mechanical Cleaving of Optical Fibres and Waveguides

The structure of a typical optical fibre is shown in FIG. 1 of theaccompanying drawings. In a number of applications in fibre-opticcommunications it is necessary to couple light either into or out ofoptical fibres or waveguides. Sometimes this is accomplished usingconnectorised fibres where the fibre is attached into a ferrule and thenpolished to provide an optical quality surface with the end of the fibrelying approximately flush with the end of the ferrule. However, in otherapplications, the fibre is not connectorised. In these cases, mechanicalcleaving of optical fibres is the accepted technique for preparing theends of the fibre. This is also the case when the ends of the fibre needto be prepared prior to mechanical or fusion splicing.

Mechanical cleaving involves producing a fiducial stress-raising mark onthe periphery of the fibre (typically with a diamond blade), and thensnapping the fibre from the mark. When carried out correctly, thisleaves a high quality, flat surface across the vast majority of the endof the fibre, including across the crucial core region.

In many applications it is required to minimise the optical reflectionfrom the end face of the fibre or waveguide back into the fibre orwaveguide. This can be achieved by angling the end face of the fibre orwaveguide (as shown in FIG. 1) so that the back-reflected light isreflected away from the core. The greater the angle, the less light iscoupled back into the core of the fibre or waveguide. Typically anglesof 6–8° are used which are close to the limit of what can be obtainedwith a degree of reliability in mass production.

In the interests of increasing the component density in opto-electronicdevices, however, laser sources which emit vertically (normal to theplane of the chip rather than parallel to it, see FIGS. 2( a) and (b))are being developed. Coupling the light from these sources into opticalfibres or waveguides presents a challenge to conventional techniques,but can be accomplished using total internal reflection form an end facecleaved at approximately 45° to the fibre or waveguide axis as shown inFIG. 2( b).

Mechanical cleaving has a number of disadvantages and limitations.Firstly, it produces very sharp edges on the corner of thee cleaved(cut) fibre. These are susceptible to handling damage, particularly ifthe fibre is to be inserted longitudinally into another component.

In some cases these sharp edges are removed in a second process, forexample by introducing the tip of the fibre into a flame.

Secondly, the range of angles which mechanical cleaving can achieve islimited. Devices relying on stressing the fibre during the cleaveprocess (either by twisting the fibre or by applying a shearing stress)result in an angle on the cleaved end but in practice this is restrictedto <10°. Angles of close to 45° required for coupling light into thefibre or waveguide from vertical emitting lasers by means of areflection from the end face of the fibre or waveguide (see FIG. 2)cannot be achieved. Moreover, the reproducibility of the cleave angle isless than is called for in many applications, with ±0.5° being difficultto maintain in mass production whereas ±0.2° is often desired.

Thirdly, as mechanical cleavers depend for their operation on precisionmoving parts including a very sharp blade, they are prone to wear andmisalignment, requiring more readjustment and refurbishment than isideal for mass production.

Fourthly, the mechanical cleave process, involving such precise andintimate contact between the cleaver and the fibre, is inherentlydifficult to automate. Such a non-automated process requiresconsiderable manpower resources to produce large volumes, and the yieldis dependent on operator skill which leads to product variability.

Fifthly, the size of the hardware involved in the mechanical cleavemeans that is not possible to cleave very close to other objects. Forexample, cleaving cannot generally be carried out closer than about 10mm from a ferrule or connector.

Further, mechanical cleaving cannot produce the very tightfibre-to-fibre cut length tolerances required of ribbon fibres, wheretolerances of ±2 μm or less are required.

2. Producing Lens-Shapes on Optical Fibres and Waveguides

Increasing data traffic is placing ever greater demands on theperformance of optical communications systems. These include capacity,bandwidth and distance between amplifiers or repeaters.

Crucial to meeting the above objectives is to maximise the efficiency ofthe whole system. This not only reduces the power consumed and/or allowsthe use of fewer amplifier/repeaters, but results in less waste heat andhence thermal loading of the components. This reduces the thermalmanagement hardware needed, permits tighter packaging of components, andallows the active devices to be operated at lower temperatures, whichhas a significant beneficial effect on component lifetimes.

One significant source of inefficiency in a pig-tailed transmitter orpump laser is the coupling of the emitted laser power into the attachedfibre. The problem here is to couple the divergent optical output fromthe laser diode, which will have an effective source size of a fewmicrons and usually different beam divergences in the two orthogonaldimensions, into the (usually) circularly symmetric core of an opticalfibre or waveguide which, for a single mode fibre or waveguide, will bebetween 3 and 20 μm in diameter, or may be up to 62 μm or more formulti-mode fibre or waveguides.

The optical transfer from the source to the fibre or waveguide is oftenaccomplished using micro-optics inserted between the two components asshown in FIG. 2( c). The production and alignment, assembly andsubsequent permanent fixturing of these discrete components isproblematic. For reasons of availability and ease of alignment, thelenses are often spherical and symmetric, although it is clear thataspheric, asymmetric lenses would provide superior performance.

Producing a lens-shape directly on the end of the optical fibre orwaveguide can reduce the alignment difficulties by avoiding the need forthe additional (aligned) component. Various techniques for manufacturingsuch a lens have been described, including etching, selective etching(where the cladding is selectively removed and the core then etched),grinding, pulling the fibre in the presence of a heat source (usually anelectric arc) and laser micro-machining.

The laser route has a number of advantages in terms of speed,flexibility and reproducibility.

The use of a CO₂ laser to machine lens shapes on optical fibres by meansof a micro-lathe approach has been described in a number of patents (forexample, see U.S. Pat. No. 4,710,605, EP 0 391 598 B, EP 0 558 230 B).In these patents, the laser is focused to a spot, which is then scannedacross the end of the rotating fibre, providing a machining approachwhich is analogous to a conventional mechanical lathe.

This approach introduces a significant heat input into the fibre. Thisresults in a re-flow of material which is influenced by surface tensioneffects. The net result is a smoothing of fine detail and a tendencytoward smoothly curved and ultimately near-spherical surfaces. For thepurposes of these patents, this is a largely helpful phenomenon whenproducing relatively gently curved lenses with tip radii (assuming thespherical case) in excess of 10 μm. However, production of radii lessthan 10 μm is problematic with the micro-lathe technique.

Moreover, in practice the technique is relatively slow (of order 15 sper fibre), and tends to “flare” the fibre, causing the fibre outsidediameter (OD) to locally increase beyond the nominal 125 μm, as shown inFIG. 3( a). This is a severe disadvantage if it is wished to passivelyalign the fibre to an active device (say a laser source) by laying thefibre in a v-groove (FIG. 3( b)). In such an application, the toleranceon alignment is typically of order 0.3 μm, and so even 1 μm levels offlare have a significant detrimental effect.

In addition, the significant thermal input in the process describedabove can result in diffusion of the dopant which defines the core andhence the active region of the fibre (see FIG. 4). This core diffusioncan have a deleterious effect on the optical performance of the lens.

Furthermore, the significant thermal input can cause severe problemswhen machining polarisation maintaining (PM) fibre, which typically haveasymmetrically distributed inserts of a different or doped materialwithin the fibre to provide stress directions and hence the PM axis.This different material will generally have different thermal propertiesto the surrounding quartz, in particular it will melt and re-solidify ata different (usually lower) temperature. If the laser lensing processproduces a significant melt region, as the micro-lathe does, the effectsof different parts of the end face of the fibre re-solidifying atdifferent times can severely distort the overall surface form.

3. Cutting of Optical Fibres with a Laser

The use of lasers to cut optical fibres has also been described. U.S.Pat. No. 5,421,928 (Siecor Corporation) describes a method in which afocussed laser beam is used to cut excess optical fibre protruding froma ferrule prior to polishing, and EP 0 987 570 A (The WhitakerCorporation) describes a process in which a focussed laser beam istranslated across a fibre in order progressively to cut through thefibre (a similar technique is disclosed in U.S. Pat. No. 4,932,989).

OBJECTS AND SUMMARY OF THE INVENTION

The principal object of the present invention is to overcome or at leastsubstantially reduce some at least of the abovementioned problems.

It is an object of the present invention also to provide an improvedlaser cutting method which is precise, reliable and reproducible duringoperation for optical fibre and optical waveguide applications.

It is also an object of the present invention to provide a simple,robust, and cost-effective route to laser machining fibres employing nomoving parts and the smallest laser size practicable.

It is another object of the present invention also to provide animproved laser cutting method which is capable of achieving one or moreof a reduction in the volume of molten fibre or waveguide materialproduced, a low interaction time to substantially prevent thermalconduction/diffusion effects, a low fibre or waveguide end flare, asharp cutting action and a high local laser intensity at the fibreportion or waveguide to be cut.

Further, it is another object of the present invention also to providean improved method of forming a lens on optical fibre or waveguide whichis precise and reliable, and which is capable of achieving in acontrollable manner one or more of low thermal conduction/diffusioneffects, a low optical absorption depth of laser in the fibre orwaveguide material, a sharply curved lens-shape (for example, with atight lens tip radius), a fast processing of the fibre or waveguidematerial, a low fibre or waveguide end flare and a low core diffusion.

In broad terms, the present invention in one of its aspects resides inthe concept of taking advantage of the sharp cutting edge of apredetermined laser beam intensity profile to provide a controllableablation and vaporisation of selected optical fibre or waveguidematerial, such ablation and vaporisation enabling a cutting action ofthe type which meets the aforementioned objects to be effected.

Therefore, according to a first aspect of the present invention there isprovided a method of cutting a portion of an optical fibre or waveguidewith a stationary laser beam, the method comprising: generating a beamof laser light with a predetermined intensity distribution whichexceeds, in one dimension, the width of the fibre or waveguide, aligningsaid beam distribution in relation to a portion of an optical fibre orwaveguide to permit an amount of said beam exceeding a predeterminedlevel of intensity to impinge on said portion to be cut; and cuttingsaid portion in dependence upon the impingement of said beam on saidportion so as to form a substantially flat optical fibre or waveguidesurface of enhanced quality.

According to a second aspect of the present invention, there is provideda method of forming a lens at an end portion of an optical fibre orwaveguide with a stationary laser beam, the method comprising:generating a beam of laser light with a predetermined intensitydistribution; aligning said beam distribution in relation to an endportion of an optical fibre or waveguide to permit an amount of saidbeam exceeding a predetermined level of intensity to impinge on said endportion; and cutting said end portion in dependence upon the impingementof said beam on said end portion so as to form a lens of enhancedoptical quality at said end portion.

The present invention also extends to an apparatus adapted and arrangedto carry out the aforementioned methods, said apparatus comprising:means for generating a beam of laser light with a predeterminedintensity distribution; means for aligning said beam distribution inrelation to a portion of an optical fibre or waveguide to permit anamount of said beam exceeding a predetermined level of intensity toimpinge on said portion to be cut; and means for cutting said portion independence upon the impingement of said beam on said portion so as toform an optical fibre or waveguide surface of enhanced quality, forexample a substantially flat optical surface or a lens of enhancedquality at said portion.

In yet another of its aspects, the present invention resides in theconcept of utilising means other than focussing of the laser beam toprovide the desired sharp cutting edge of a predetermined laser beamintensity profile, enabling a cutting action of the type which meets theaforementioned objects to be effected.

More particularly according to this aspect of the present inventionthere is provided a method of cutting a portion of an optical fibre orwaveguide with a laser beam, the method comprising: generating a beam oflaser light with a predetermined intensity distribution other than byfocussing, aligning said beam distribution in relation to a portion ofan optical fibre or waveguide to permit an amount of said beam exceedinga predetermined level of intensity to impinge on said portion to be cut;and cutting said portion in dependence upon the impingement of said beamon said portion.

This aspect of the present invention also extends to an apparatusadapted and arranged to carry out the aforementioned method, saidapparatus comprising: means for generating a beam of laser light with apredetermined intensity distribution other than by focussing, means foraligning said beam distribution in relation to a portion of an opticalfibre or waveguide to permit an amount of said beam exceeding apredetermined level of intensity to impinge on said portion to be cut;and means for cutting said portion in dependence upon the impingement ofsaid beam on said portion.

Conveniently, by utilising means other than focussing of the laser beam,high intensity laser cutting beam distributions well suited to thecutting process can be produced. In this connection, the desiredgenerated beam distribution can be formed by optical interference,imaging or diffraction or by a combination of such techniques.

Having regard to the foregoing, it is to be appreciated that themethod(s) and apparatus of the aforementioned aspects of the inventionhave definite advantages over known cutting methods and apparatuses; forexample, the method(s) and apparatus of the invention address thelimitations of the mechanical cleaving (cutting) route.

First, and more particularly, the laser cutting action of the inventionproduces fibre or waveguide corners which are rounded, and therefore,are more robust.

Secondly, the angle of the cut/cleave is governed by simple geometricconsiderations between the fibre or waveguide and the cutting laserbeam, and so can reach large angles (certainly >45°). Moreover, thecut/cleave angle reproducibility reflects the reproducibility of thisgeometry, and can easily better the ±0.5 degree of reproducibilityobtained by mechanical cutters/cleavers and indeed the ±0.2 degree ofreproducibility specified in some applications.

Thirdly, as the laser route is a non-contact process, there are noblades or mechanically stressed moving parts to wear out or becomemisaligned.

Fourthly, being non-contact, the laser cutting action of the inventionis inherently suited to automated loading and unloading of the fibre orwaveguide.

Fifthly, as the cutting is carried out by the laser beam itself and thehardware is remote from the cutting point, the cut can be positionedvery close to other components, certainly <1 mm.

Sixthly, precision stepping methods exist through which the cutting beamcan be stepped from fibre to fibre in a ribbon, giving fibre-to-fibrecut/cleave length variations of <1 μm.

Further, the laser cutting of the present invention combines severaltechniques designed to reduce the undesirable thermal effects of theconventional laser micro-lathe approach. These include, as mentionedpreviously, minimising the volume of molten material produced, keepingthe overall interaction time as short as possible to prevent thermalconduction/diffusion and fibre or waveguide flaring, minimising theoptical absorption depth of the laser radiation in the material, usingthe sharpest feasible “cutting edge” to the laser beam, and employingconstructive interference to enhance the local laser intensity, therebyallowing shorter interaction times.

Advantageously, keeping the quantity of molten material produced duringall stages of the interaction to a minimum minimises the undesirableeffects of the re-flow of this melted volume. It also minimises thepotential transport of the fibre or waveguide dopant from the coreregion into the cladding (refer to FIG. 4). Note also that keeping thelaser interaction time to a minimum reduces the thermal conduction fromthe region which is directly heated by the laser into other parts of thefibre or waveguide, thereby reducing the overall thermal impact.

Further, the lens forming method of the present invention combinesseveral techniques designed to reduce the undesirable thermal effectsinherent in the conventional laser micro-lathe approach, allowing, aspreviously mentioned, the achievement of tighter tip radii, fasterprocessing, minimising fibre flare and core diffusion, and providing theability to handle polarisation maintaining (PM) fibre. It also avoidsthe need to move the laser beam into and through the optical fibre asspecified in EP 0 391 598 B1 for example.

The above and further features of the invention are set forth withparticularity in the appended claims and will be described hereinafterby reference to exemplary embodiments shown in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional optical fibre structure with an angled endface to back-reflect light away from the fibre core;

FIG. 2 shows three conventional laser-to-optical fibre geometries inwhich (a) the laser source emits light parallel to the longitudinal axisof the fibre, (b) the laser source emits light vertically, normal to thelongitudinal axis of the fibre and (c) the laser source and the fibreare optically coupled by means of a lens which can be formed directly onthe end of the fibre.

FIG. 3( a) shows an optical fibre with an increased outside diameter(flare) produced by conventional laser machining treatment and FIG. 3(b) shows the problem of aligning the fibre of FIG. 3( a) in a v-shapedgroove in relation to an active source, a laser for example;

FIG. 4 shows the fibre of FIG. 3 with an unwanted amount of fibre dopantdiffusion from the core region into the cladding resulting fromconventional laser cutting of the fibre;

FIG. 5 shows, in schematic form, how a cutting action according to anaspect of the present invention is effected having regard to therelative alignment between a laser having a predetermined intensitydistribution and the optical fibre region to be cut;

FIG. 6 shows a conventional laser cutting geometry in which a laserincision is made in a moving workpiece to produce a slot/cut;

FIG. 7( a) shows, schematically, a conventional optics-geometry forproducing a line-focus laser beam with the beam focussed in twodifferent axial positions in orthogonal axes and FIG. 7( b) shows,schematically, a typical optics-geometry of the invention for producinga high intensity asymmetric line-focus beam at one axial position;

FIG. 8 shows two types of predetermined laser intensity distribution foruse in the invention, namely (a) a Gaussian intensity distribution and(b) an Airy-type intensity distribution;

FIG. 9 shows two types of mask geometry namely, (a) a rectangular maskand (b) a knife-edge mask for use in the practice of the presentinvention;

FIGS. 10( a) and (b) show, schematically, how the local laser beamintensity at the fibre portion to be cut may be enhanced by means ofconstructive interference between reflected and non-reflected parts ofthe laser beam;

FIGS. 11( a) and (b) show, schematically, how a cutting action may beeffected in accordance with the present invention by means of a numberof successive cutting steps so as to reduce the thermal loading on thefibre to be cut;

FIG. 12 shows schematically (a) a rectangular mask geometry for use inthe invention and (b) how the mask geometry of (a) is used to produce anincrease in the laser beam intensity on the fibre by opticaldemagnification;

FIG. 13 shows schematically a curved mask geometry for use in thepractice of the invention to produce an optical lens structure on theend of the fibre; and

FIG. 14 shows schematically an optical interference geometry for use inthe practice of the invention for producing a high local laser beamintensity.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 5 illustrates, in schematic form, how the cutting action of amethod of the invention is effected on a portion of optical fibre orwaveguide so as to form an optical fibre or waveguide surface ofenhanced quality. More specifically, as shown, the cutting action isaffected by impinging an amount 1′ of a predetermined laser intensitydistribution 1, for example in the form of a Gaussian intensitydistribution 1, onto a section 2 (to be cut) of fibre or waveguidematerial 3 in alignment therewith and ablating or vaporising the fibreor waveguide section 2. Regions 4 of the fibre, adjacent to the fibresection 2, are further subjected to significant levels of heating independence upon the impingement, the regions 4 being heated to anappropriately significant level below the fibre or waveguide material'svaporisation point.

Advantageously, the laser cutting procedure of the invention does notinvolve the production of a mark on the fibre periphery and thenmechanical fracture of the fibre as required by the conventionalcleaving technique. Note also that in the method of the invention, toreduce/minimise the amount of molten fibre or waveguide region 4adjacent to the vaporised region 2, the laser beam is absorbed within athin layer of the fibre or waveguide material 3, and the laser beamprofile 1 is arranged to have the sharpest possible cutting “edge”. inoperation of the method of the invention, a CO₂ laser (not shown) ispreferably used to generate the laser-beam profile, and advantageously,the CO₂ laser wavelength is changed to 9.4–9.8 μm from the more usual10.6 μm to permit the optical absorption in the quartz of the fibre orwaveguide to be increased by almost an order of magnitude, resulting ina corresponding decrease in the optical absorption depth.

Turning next to FIG. 6, there is shown, for comparison purposes, aconventional laser cutting arrangement in which a cut is performed on aworkpiece 10 by using a laser focus to provide a laser “incision” in theworkpiece 10 and then moving either the laser focus or the workpiece totranslate this incision producing a narrow slot or cut 11, as is shownin FIG. 6. This is analogous to the use of a jig-saw in woodworking.This approach can lead, however, to significant thermal input. Thepresent invention is different in principle to this mode of operation,and uses an asymmetric laser beam profile to perform the cutting. Thismore closely resembles the use of a chisel in the woodworking analogy,and results advantageously in a much more rapid process, leading to asignificantly reduced thermal impact.

The most common route to produce an asymmetric laser beam is toincorporate a single cylindrical element 15 in the optical train, oftencombined with conventional circularly-symmetric (spherical or aspheric)optics 16. This indeed produces a line-focus, however the beam isfocused in different axial positions 17, 18 in orthogonal axes as shownin FIG. 7( a), leading to a focal intensity not as high as is possible.In order to reach higher intensity a cylindrical telescope can be usedto expand or contract the laser beam in one dimension only. As is shownin the embodiment of FIG. 7( b), a cylindrical telescope 20 is combinedwith a symmetric (spherical or aspheric) focussing optic 22, and theresulting different numeral aperture in two axes produces a single highintensity line focus 25 ideally suited to laser cutting. Moreover, asthe focus in the two orthogonal axes remains in the same position alongthe propagation axis with this arrangement, the intensity in the focalline is maximised. This yields the highest optical intensity, with thesharpest “edge” to the beam allowed by free-space Gaussian beampropagation, and allows the shortest possible interaction time to beused which reduces heat conduction, melt depth and hence reduces flaringand core diffusion.

It is appreciated that the laser cutting of the invention is carried outby appropriate apparatus using the edge of the incident predeterminedlaser power density distribution (see also FIG. 5). It is possible tosharpen this edge beyond the limits which Gaussian optics wouldgenerally impose by employing a mask to aperture the beam. This resultsin an Airy-type power density distribution 28 having sharper edges(albeit with additional diffraction structure 28′) as shown in onedimension in FIG. 8. A Gaussian power density distribution 27 is alsoshown in FIG. 8 for comparison. Further, the essentially 1-D geometry ofthe laser cut means that a mask with different properties in twoorthogonal dimensions is most appropriate. Preferably a rectangular mask30 is used as shown in FIG. 9. An alternative possibility, giving asomewhat modified focal distribution, is to use a single sided mask, forexample a knife-edge 31, and that kind of mask is also shown in FIG. 9.

To further maximise the local intensity at the fibre or waveguide to becut, the laser beam may be caused to propagate in such a direction thatthe partially cleaved (cut) fibre or waveguide reflects the laser beamin such a way that the reflected beam constructively interferes with thenon-reflected part of the beam. This effect is illustrated in FIGS. 10(a) and 10(b). In this way, the local amplitude is doubled (assumingperfect reflection), resulting in a four-fold increase in the localintensity. This requires that the polarisation of the laser beam isparallel to the cut surface (rather than perpendicular) and that theangle of incidence (as measured from the normal top the surface) is lessthan a critical angle at which a π(180°) phase shift occurs onreflection of the beam.

The thermal loading on the fibre or waveguide is advantageously reducedby allowing all but the cutting edge of the laser beam to pass by thefibre or waveguide in use. This is achieved by carrying out the lasercut(s) in accordance with the aforementioned geometry of FIGS. 10( a)and 10(b), as opposed to the alternative approach shown in FIG. 11( b)(top schematic) where the laser beam propagation is shown to be directedinto the plane of the paper. The thermal loading on the fibre orwaveguide can still further be advantageously reduced by utilising thegeometry of FIGS. 11( a) and 11(b) (bottom schematic). As shown, inthese figures, a preliminary laser cut 35, 35′ (cut 1) in the normaldescribed way is performed on the fibre or waveguide portion 37, 37′ tobe treated, and thereafter, one or more subsequent laser cuts 36, 36′are performed, there being slight relative movement (not shown) betweenthe fibre or waveguide and the laser beam along the longitudinal axis ofthe fibre or waveguide between successive cuts.

Advantageously, the laser cutting procedure of the invention provides ahigh intensity beam with the sharpest possible cutting edge, enablingcore diffusion and fibre flaring to be minimised and producing the mostaccurate cut end on the fibre. It is to be appreciated that, whereaswith simple focussing, a Gaussian intensity distribution is the bestlikely distribution to be achieved, the intensity distribution for usein the present invention may advantageously be produced by means otherthan by focussing, for example by imaging, optical interference ordiffraction, or by a combination of such techniques. Further, it is tobe noted that the laser cutting of the invention is carried out byappropriate apparatus using the cutting edge of the generated laserpower density distribution.

Turning next to FIG. 12, an advantageous laser intensity distributionfor use in the practice of the present invention may be produced byimaging a rectangular mask 40 in the image plane by use of a lens 41. Asshown, the arrangement provides optical demagnification of the laserbeam in order to increase the local intensity of the laser beam on thefibre to be cut. The present invention, in this aspect, is thereforedifferent in principle to conventional modes of operation, and uses alaser beam profile with a sharp cutting edge, produced by means otherthan by focussing, to perform the cutting. This more closely resemblesthe use of a chisel in the woodworking analogy, and resultsadvantageously in a much more rapid process, leading to a significantlyreduced thermal impact.

FIG. 13 shows a different mask geometry from that of FIG. 12 for use inthe present invention. More particularly, as shown, a mask geometry witha curve-shaped cut 42 is used to provide a varying level of lighttransmission such as to permit a particular optical structure such as alens to be formed on the end of a fibre. Note that the intensitydistribution in the image (machining) plane is altered by the way inwhich the object (mask) is illuminated by the laser, which need not beuniform.

FIG. 14 shows another optical arrangement of the present invention inwhich optical interference is used to produce the high local intensitydesired. As shown in the figure, an optical interference arrangement 50similar to the so-called Lloyds Mirror in classical optics is used. Thisis used with a point/slit source 51 and gives rise to a number ofparallel fringes 52. For the purposes of the present invention, it maybe used to produce the high local intensities desired. With correctcontrol over the divergence of the laser beam (and/or the effectivesource size), the majority of the photons are advantageouslyconcentrated into the first fringe, with little energy wasted in thehigher order fringes. The resulting intensity pattern is highlyconcentrated, yielding the high intensities required.

Other wavefront-splitting interferometers (not shown) are known inclassical optics which produce fringe patterns similar to the Lloyd'smirror arrangement, and which are similarly adapted to optical fibrecutting in accordance with the present invention. These includeFresnel's double mirror arrangement, Young's slits arrangement andFresnel's Bi-prism arrangement.

In another optical arrangement (not shown) of the present invention theconstructive interference of phase-shifted beams is used to produce therequired local high intensity. This phenomenon is known in classicaloptics, but has not been employed in laser machining. Diffraction of thecutting beam using phase masks, zone plates and/or echelle gratings isenvisaged so as to effect fibre cutting in accordance with the presentinvention.

Having regard to the foregoing, the laser cutting action proposed by thepresent invention finds utility for various optical fibre or waveguideapplications. In this connection, it permits a selected region of anoptical fibre or waveguide to be cut in a controlled fashion enablingvarious cut angles (≦to >45 degrees) to be formed in the fibre orwaveguide, and as described above, it can be successfully applied tooptical fibre or waveguide to form, for example, (1) a substantiallyflat optical fibre or waveguide surface of enhanced optical quality or(2) a lens of enhanced optical quality at said region. The laser beamduring the cutting operation is kept at a stationary position inrelation to the fibre or waveguide.

Having described the invention by reference to specific embodiment, itis to be well understood that the embodiments are exemplary only andthat modifications and variations thereto will occur to those possessedof appropriate skills without departure from the spirit and scope of thepresent invention as set forth in the appended claims. For example,whereas the described embodiment of the invention uses a Gaussian orAiry-type (non Gaussian) laser intensity distribution, the same orsimilar technical effect might be obtainable by using a different kindof non-Gaussian laser intensity distribution having a sharp cuttingedge. Also, the laser source need not be a CO₂ laser as in the describedembodiment and could alternatively be a UV-excimer laser. The inventioncan also be applied to different optical fibre or waveguide structureswhere a sharp cutting action is required. It is also to be appreciatedthat a very wide range of laser intensity distributions might beproduced using different kinds of masks having different forms andshapes. Different masks having varying levels of transmission can beused to produce the same or similar technical effect.

1. A method of cutting a portion of an optical fibre or waveguide with alaser beam, the method comprising: generating a beam of laser lighthaving a predetermined intensity distribution which is wider than thewidth of the fibre or waveguide at the point of contact between thelaser beam and fiber or waveguide, aligning said beam distribution inrelation to a portion of said optical fibre or waveguide to permit anamount of said beam exceeding a predetermined level of intensity toimpinge on said portion to be cut; and cutting said portion independence upon the impingement of said beam on said portion so as toform a substantially flat optical fibre or waveguide surface of enhancedquality.
 2. A method as claimed in claim 1, wherein said cutting iseffected by ablating or vaporising said portion and heating adjacentportions of the fibre or waveguide.
 3. A method as claimed in claim 1,wherein the generated beam distributed comprises an asymmetric beam. 4.A method as claimed in claim 3, wherein asymmetric beam is formed by useof cylindrical telescope means.
 5. A method as claimed in claim 1,wherein said predetermined intensity distribution is a Gaussianintensity distribution.
 6. A method as claimed in claim 1, wherein saidpredetermined intensity distribution is an Airy-type intensitydistribution.
 7. A method as claimed in claim 1, wherein saidimpingement of the beam on said portion is effected in a manner topermit constructive interference between reflected and non-reflectedparts of said beam.
 8. A method as claimed in claim 1, furthercomprising effecting relative movement between said portion and the beamafter a first cutting operation and thereafter performing a second cut.9. A method as claimed in claim 8, wherein said relative movementcomprises a small predetermined movement along the longitudinal axis ofsaid fibre or waveguide.
 10. A method as claimed in claim 8, furthercomprising cutting said portion a third or more time.
 11. An apparatusadapted and arranged to carry out a method as claimed in claim
 1. 12. Anapparatus as claimed in claim 11 comprising: means for generating a beamof laser light with a predetermined intensity distribution; means foraligning said beam distribution in relation to a portion of an opticalfibre or waveguide to permit an amount of said beam exceeding apredetermined level of intensity to impinge on said portion to be cut;and means for cutting said portion in dependence upon the impingement ofsaid beam on said portion so as to form an optical fibre or waveguidesurface of enhanced quality.
 13. An apparatus as claimed in claim 12,wherein said means for generating a beam of laser light is a CO₂ laseroperable in the wavelength range between 9.4 μm and 10.6 μm.
 14. Theapparatus as claimed in claim 12, wherein said means for cutting forms aflat optical surface on said optical fibre or waveguide surface.
 15. Anapparatus as claimed in claim 12, wherein said means for generating abeam of laser light is a CO₂ laser operable in the wavelength rangebetween 9.4 micrometers and 9.8 micrometers.
 16. The apparatus of claim12, wherein said means for cutting forms a lens on said optical fibre orwaveguide surface.
 17. A method of forming a lens at an end portion ofan optical fibre or waveguide with a stationary laser beam, the methodcomprising: generating a beam of laser light with a predeterminedintensity distribution; aligning said beam distribution in relation toan end portion of an optical fibre or waveguide to permit an amount ofsaid beam exceeding a predetermined level of intensity to impinge onsaid end portion; and cutting said end portion in dependence upon theimpingement of said beam on said end portion so as to form a lens ofenhanced optical quality of said end portion, wherein a width of thebeam is sufficient for cutting to be effected without relative movementbetween the laser beam and the fibre or waveguide.
 18. A method ofcutting a portion of an optical fibre or waveguide with a laser beam,the method comprising: generating by means other than focusing a beam oflaser light having a predetermined intensity distribution that is widerthan a width of the fibre or waveguide at a point of contact betweensaid laser and fibre or waveguide; aligning said beam distribution inrelation to a portion of an optical fibre or waveguide to permit anamount of said beam exceeding a predetermined level of intensity toimpinge on said portion to be cut; and cutting said portion independence upon the impingement of said beam on said portion, whereinsaid cutting step is achieved without relative movement between thelaser beam and the optical fibre or waveguide.
 19. A method as claimedin claim 18, wherein said cutting is effected by ablating or vaporisingsaid portion and heating adjacent portions of the fibre or waveguide ata predetermined level.
 20. A method as claimed in claim 18, wherein thegenerated beam distribution is formed by imaging.
 21. A method asclaimed in claim 18, wherein the generated beam distribution is formedby interference.
 22. A method as claimed in claim 21, wherein saidgenerated beam distribution is formed by use of an optical interferenceLloyd's mirror geometry, or by use of an optical interference Fresneldouble mirror geometry, or by use of an optical interference FresnelBi-prism geometry, or by use of a Young slit's geometry.
 23. A method asclaimed in claim 18, wherein the generated beam distribution is formedby diffraction.
 24. A method as claimed in claim 23, wherein saidgenerated beam distribution is formed by use of mask means.
 25. A methodas claimed in claim 24, wherein said mask means comprises a phase maskor zone plate.
 26. A method as claimed in claim 23, wherein saidgenerated beam distribution is formed by use of an echelle grating. 27.A method as claimed in claim 18, wherein said generated beamdistribution is formed by means of optical interference and/or imagingand/or diffraction.
 28. An apparatus adapted and arranged to carry out amethod as claimed in claim
 18. 29. An apparatus as claimed in claim 28comprising: means for generating a beam of laser light having apredetermined intensity distribution other than by focusing; means foraligning said beam distribution in relation to a portion of an opticalfibre or waveguide to permit an amount of said beam exceeding apredetermined level of intensity to impinge on said portion to be cut;and means for cutting said portion in dependence upon the impingement ofsaid beam ion said portion.
 30. An apparatus as claimed in claim 29,wherein said means for generating a beam of laser light is CO₂.
 31. Anapparatus as claimed in claim 30, wherein cutting of the optical fibreor waveguide is effected without translation of the laser beam acrossthe fibre or waveguide.
 32. A method of cutting a portion of an opticalfibre or waveguide with a stationary laser beam, the method comprising:generating a beam of laser light with a predetermined intensitydistribution, aligning said beam distribution in relation to a portionof a stationary optical fibre or waveguide to permit an amount of saidbeam exceeding a predetermined level of intensity to impinge on saidportion to be cut; and cutting said portion in dependence upon theimpingement of said beam on said portion so as to form a substantiallyflat optical fibre or waveguide surface.